Disc drives are data storage devices that store digital data on a rotating disc. Modern disc drives comprise one or more rigid information storage discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers mounted to a radial actuator for movement of the heads relative to the discs. During a data write operation sequential data is written onto the disc track, and during a read operation the head senses the data previously written onto the disc track and transfers the information to an external environment. Important to both of these operations is the accurate and efficient positioning of the head relative to the center of the desired track on the disc. Head positioning within a desired track is dependent on head-positioning servo patterns, i.e., a pattern of data bits recorded on the disc surface and used to maintain optimum track spacing and sector timing. Servo patterns or information can be located between the data sectors on each track of a disc (“embedded servo”), or on only one surface of one of the discs within the disc drive (“dedicated servo”). Regardless of whether a manufacturer uses “embedded” or “dedicated” servos, the servo patterns are typically recorded on a target disc during the manufacturing process of the disc drive.
Recent efforts within the disc drive industry have focused on developing cost-effective disc drives capable of storing more data onto existing or smaller-sized discs. One potential way of increasing data storage on a disc surface is to increase the recording density of the magnetizable medium by increasing the track density (i.e., the number of tracks per inch). Increased track density requires more closely-spaced, narrow tracks and therefore enhanced accuracy in the recording of servo-patterns onto the target disc surface. This increased accuracy requires that servo-track recording be accomplished within the increased tolerances, while remaining cost effective.
Servo patterns are typically recorded on the magnetizable medium of a target disc by a servo-track writer (“STW”) assembly during the manufacture of the disc drive. One conventional STW assembly records servo pattern on the discs following assembly of the disc drive. In this embodiment, the STW assembly attaches directly to a disc drive having a disc pack where the mounted discs on the disc pack have not been pre-recorded with servo pattern. The STW does not use any heads of its own to write servo information onto the data surfaces, but uses the drive's own read/write heads to record the requisite servo pattern to mounted discs.
In light of the explosive trend toward higher track densities in recent years, some exceeding 100,000 tracks per inch, this conventional method has become excessively time consuming. As the trend continues, it will apparently be necessary for every disc drive manufacturer to obtain and operate much larger numbers of STW's to maintain comparable numbers of disc drives. One strategy to mitigate this need is to utilize multi-disc “ex situ” STW's, which are are capable of recording servo patterns to multiple discs mounted in a stack. After writing some of the position information using (dedicated) servo recording heads, sequentially or simultaneously, the discs are then removed and loaded into disc drives for use.
With any of these methods, substantial errors in the shapes and/or positions of tracks remain. Seagate Technology has devised several compensation systems that seek to correct such errors so that a head can follow a more circular path while reading and writing data. These systems rely on large numbers of embedded correction values called “Zero Acceleration Path” (ZAP) correction factors. Fully implemented, ZAP correction factors are very effective for adjusting track shapes. Unfortunately, such an implementation may cost too much, in light of current trends toward higher track density.
What is needed is an efficient system for making use of coherent runout, meaning written-in error that is very similar in phase and magnitude from track to track, at least over a localized region of a disc surface. Because coherent runout often accounts for a large portion of written-in errors in servo marks, such a system should improve disc drive performance, reduce manufacturing costs, or both.